Eric Allendery
Department of Computer Science
Rutgers University
New Brunswick, NJ 08903
Ulrich Hertrampf
Institut fur Informatik
Universitat Wurzburg
D-8700 Wurzburg
Federal Republic of Germany
December 12, 1991
The results in this paper were originally announced in papers in Proc. 30th IEEE Sympo-
sium on Foundations of Computer Science 1989 and in Proc. 15th International Symposium
on Mathematical Foundations of Computer Science 1990.
ySupported in part by National Science Foundation grants CCR-8810467and CCR-9000045.
Some of this research was performed while the rst author was a visiting professor at Institut
fur Informatik, Universitat Wurzburg.
1